Polymers always have a residual content of monomers from which they are synthesized, and an inherent color. The removal of the residual monomers from the molding compositions frequently involves exposure to high temperatures, which leads to an undesirable intensification of the inherent color. In order to improve the properties it is essential to remove the residual monomers in a suitable way.
In practice a light inherent color is generally desirable. In the case of colored products this also generates an economic benefit, because the amount of dye needed for coloration may be reduced rather than having to be increased in order to conceal a yellowish inherent color.
A removal of residual monomers by the chemical route is described for example in EP-A1-0 768 337. Here the residual monomers are removed by addition of CH-acid organic compounds. Through this known process monomers such as styrene and acrylonitrile can be removed reliably, but not solvents or secondary polymerization products. The same deficiency occurs in the process to reduce residual monomers with alkali (bi)sulfide (DE-A1-2 546 182). In some circumstances, however, the chemical transformation of residual monomers leads to products having an undesirable ecological relevance, which significantly complicates the use of these products in practice.
In another known process the residual monomers are removed under vacuum in an additional process step in extruders with or without the addition of water. The additional process step leads to higher costs and to a further undesirable deepening of the product color.
A process for the removal of residual monomers by injection of supercritical solvents into the polymer melt (EP-A1-0 798 314) has proven costly.
Another process for the production of elastomer-modified thermoplastics is described in EP-A1-0 867 463. Wet polybutadiene powder grafted with styrene and acrylonitrile and containing residual monomers is blended with polystyrene-acrylonitrile melt in a mixing compounder and freed from residual monomers with simultaneous evaporation of the water. The finished ABS is discharged from the end of the evaporating equipment and converted into pellet form. The disadvantage of this process is that the achievable throughput is limited. With a high throughput the powder is to a considerable extent not incorporated into the melt but entrained with the vapors. This leads to loss of yield and to unstable operating conditions. To counteract this mode of operation of the mixing compounder can be adjusted so that a back-up of melt is generated at the exit from the mixing compounder. This melt is supposed to bind the powder and prevent it from being discharged. This procedure is only of limited use, however.
Starting from this known process, the object of the present invention is to provide a process that does not have the disadvantages described.